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I would still recommend adding a sub, however:
1. it's not as easy as you think, so there's something to learn while doing it trying crossovers, delays, phase rotations etc.
2. you won't loose any resale value, modified speakers are worth basically nothing on the used market
3. you have much bigger chance of success of it being at least listenable
4. another upside is the final result which would be better with a subwoofer or two, than anything you could do modifying the speakers
5. and most importantly you can still do digital crossover, improve the whole range and tweak the whole system to your liking without changing any driver, so it still looks the same and is easily reversible
 
Extended LF from old loudspeakers?
Stick the loudspeakers in the corners of a room.
:)
 
So, if I understood it all correctly, you have bought : To test this theory, I got my hands on a pair of 488s

Are these 488, or 22AH488 (also marketed as the 22AH468, which has a different look) fully functional?

Then you have two pairs of speakers. A pair of Philips 22AH487, with bass driver with passive radiator and a pair of Philips 22AH488/22AH468 with
double bass drivers. Is it true so far?

Your two pairs of speakers seem to have the same mid drivers and tweeters. Why not keep 22AH488/ 22AH468 as it is? Have you measured your 22AH488/22AH468?

Philips 22AH468:
unnamed.jpg

Philips 22AH487:
IMG20240928113211~2.jpg

Edit:
Or do you mean that you only got hold of new bass drivers? Four pieces or two AD8067? If you have got hold of four, why not remove the passive radiators and install the four bass drivers in your boxes? Then you have some power to play with and then EQ and raise the frequency a little at the bottom where they start to roll off. :)

Maybe you can test with a Linkwitz transform compensated bass system if you have sealed boxes? Note that I have never tested a Linkwitz transformation myself. In that case, others can give you tips and advice about that:
Screenshot_2024-12-19_153842.jpg

 
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Adding mass to the passive radiator would drop the tuning frequency. However the excursion will increase, so a single PR is likely insufficient, the speaker would have reduced peak SPL. The enclosure volume may also be insufficient, causing the PR efficiency to drop.

Bracing would almost certainly improve bass and mid-bass quality but not help bass extension.

Minor air leaks would not significantly change the bass response. They usually show up in impedance sweeps if you want to check for them.

Overall, adding a sub is the easiest and best solution. BK Elec offers wood finishes if you want to match the aesthetic without a DIY build.


You mentioned running a DSP 3 way crossover. This won't increase bass extension but will significantly improve the overall performance. Better management of driver breakup modes, no distortion or wasted power from passive crossover components, EQ to pull down room mode peaks, etc.
 
Cheers to the new year. Thanks again everyone for your advice. I did some more research these holidays and I'd like to share where I'm at right now and respond to some of your comments.

So, if I understood it all correctly, you have bought : To test this theory, I got my hands on a pair of 488s

Are these 488, or 22AH488 (also marketed as the 22AH468, which has a different look) fully functional?

Then you have two pairs of speakers. A pair of Philips 22AH487, with bass driver with passive radiator and a pair of Philips 22AH488/22AH468 with
double bass drivers. Is it true so far?
Yes, what you interpreted is correct.

I obtained two defective pairs of 22AH468 for cheap. I managed to fully repair one pair. The mids didn't work, but were easily fixable following this writeup. It's a common defect with these units. The second pair has one non-repairable blown mid and someone replaced one of the woofers with a random aftermarket unit, so I can't bring this pair back to original form without additional units sourced from somewhere.

Your two pairs of speakers seem to have the same mid drivers and tweeters. Why not keep 22AH488/ 22AH468 as it is? Have you measured your 22AH488/22AH468?

Edit:
Or do you mean that you only got hold of new bass drivers? Four pieces or two AD8067? If you have got hold of four, why not remove the passive radiators and install the four bass drivers in your boxes? Then you have some power to play with and then EQ and raise the frequency a little at the bottom where they start to roll off.

Maybe you can test with a Linkwitz transform compensated bass system if you have sealed boxes? Note that I have never tested a Linkwitz transformation myself. In that case, others can give you tips and advice about that:
The mids are the same, but the tweeters are the 15 Ohm variant (AD0163/T15), which should be about 3 dB less efficient than the 8 Ohm variant (AD0163/T8) in the 22AH487.

I measured the in-room frequency response of the now working pair 22AH468. They're okay but have a strong v-shaped sound signature. The lows have a lot of weird 'wobbly' distortion with my (admittedly sub-par) Denon UDRA-F07 amp, possibly because of the protective car lamps in the crossover filter. Bass is slightly stronger than the 22AH487s but not deeper without EQ. Probably because the 35 L box is too small for the two 8-inch AD8067/W4E woofers.

Overall, not impressed with the sound signature. And I don't dig their box design, which was my main motivator for starting this project on modifying the 22AH487 speakers.

----------

To end the speculation on what combination of drivers is most effective, I’ve decided to seek concrete answers by learning how to measure T/S parameters myself. I've researched the process, ordered the necessary components, and plan to conduct my own measurements of all the drivers with REW. Once I have the data, I'll use recommended speaker design software to evaluate whether a PR system would be effective for the AD8067/W4E, or if it would be better to simply use dual woofers with additional power compensation for lower frequencies. I can use the drivers sourced from the (defective) 22AH468 units for this.

----------

Since I'm already planning to modify the speakers anyways, I want to go all the way with improving the sonic quality of them. I.e. I've already found that the driver break-up modes are not suppressed and I doubt the drivers are phase aligned on the baffle.
  • To address panel vibrations, I’ve ordered bitumen sheets (instead of adding bracing, as I want to preserve the internal volume of the enclosures). I’ve also ordered felt for lining the walls and glass wool to dampen the back wave of the woofer(s). The goal of this isn’t to achieve deeper bass but to improve its quality, like alex-z said. The original synthetic dampening material in the 22AH487s has completely degraded and needs to be replaced anyways.
  • Additionally, I plan to follow through with an active DSP crossover for its inherent advantages and as a personal learning opportunity. To support this, I’ll add a 6-pin DIN socket, allowing individual amplifier channels to connect to each driver (or in the case of the woofer, either one, or two in series). I also consider including a triple-pole, double-throw switch to retain the option of using the original crossover, should I want to revert to the classic setup in the future. I've created a wiring diagram for this plan, but I’d appreciate it if someone could verify its accuracy:
    Wiring Diagram - Crossover + DIN.png

  • To support an active crossover project, I found several product options. Honestly, I don't really know what would be the easiest for a beginner like me, and has the best bang/buck. I can source cheaply from Chinese Taobao. I plan to use either:
    • A Raspberry Pi running CamillaDSP, combined with a Creative Sound Blaster Omni Surround 5.1 external DAC I have lying around. For amplification, I'm thinking about either:
    • 2x Wondom JAB5 boards hooked up together (master-slave), which have the ADAU1701 DSP built in and 4 amplifier channels each (4x100W). Maybe the neatest and easiest. Don't know about the quality of the DACs and amplification though.
    • I can also use an ADAU1562 DSP board with an AD1938 or CS42448 codec instead of the Raspberry Pi + multichannel DAC. Combined with one of the aforementioned amplification options.
Let me know your thoughts. :)
 
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the bass response is disappointing, especially considering the size of the enclosure... 8-inch woofer and an 8-inch passive radiator, but the bass rolls off sharply at around 50 Hz (48 dB/octave),
This behavior is typical for a passive radiator loudspeaker - it can not be made better.

My idea is to combine the best parts of the 487 and 488 designs:
Do not change drivers with different ones and do not mess with the passive radiator, PLEASE!
Just replace old bipolar electrolytic capacitors with new ones.
If you need deeper bass, buy a subwoofer.
 
Cheers to the new year. Thanks again everyone for your advice. I did some more research these holidays and I'd like to share where I'm at right now and respond to some of your comments.


Yes, what you interpreted is correct.

I obtained two defective pairs of 22AH468 for cheap. I managed to fully repair one pair. The mids didn't work, but were easily fixable following this writeup. It's a common defect with these units. The second pair has one non-repairable blown mid and someone replaced one of the woofers with a random aftermarket unit, so I can't bring this pair back to original form without additional units sourced from somewhere.


The mids are the same, but the tweeters are the 15 Ohm variant (AD0163/T15), which should be about 3 dB less efficient than the 8 Ohm variant (AD0163/T8) in the 22AH487.

I measured the in-room frequency response of the now working pair 22AH468. They're okay but have a strong v-shaped sound signature. The lows have a lot of weird 'wobbly' distortion with my (admittedly sub-par) Denon UDRA-F07 amp, possibly because of the protective car lamps in the crossover filter. Bass is slightly stronger than the 22AH487s but not deeper without EQ. Probably because the 35 L box is too small for the two 8-inch AD8067/W4E woofers.

Overall, not impressed with the sound signature. And I don't dig their box design, which was my main motivator for starting this project on modifying the 22AH487 speakers.

----------

To end the speculation on what combination of drivers is most effective, I’ve decided to seek concrete answers by learning how to measure T/S parameters myself. I've researched the process, ordered the necessary components, and plan to conduct my own measurements of all the drivers with REW. Once I have the data, I'll use recommended speaker design software to evaluate whether a PR system would be effective for the AD8067/W4E, or if it would be better to simply use dual woofers with additional power compensation for lower frequencies. I can use the drivers sourced from the (defective) 22AH468 units for this.

----------

Since I'm already planning to modify the speakers anyways, I want to go all the way with improving the sonic quality of them. I.e. I've already found that the driver break-up modes are not suppressed and I doubt the drivers are phase aligned on the baffle.
  • To address panel vibrations, I’ve ordered bitumen sheets (instead of adding bracing, as I want to preserve the internal volume of the enclosures). I’ve also ordered felt for lining the walls and glass wool to dampen the back wave of the woofer(s). The goal of this isn’t to achieve deeper bass but to improve its quality, like alex-z said. The original synthetic dampening material in the 22AH487s has completely degraded and needs to be replaced anyways.
  • Additionally, I plan to follow through with an active DSP crossover for its inherent advantages and as a personal learning opportunity. To support this, I’ll add a 6-pin DIN socket, allowing individual amplifier channels to connect to each driver (or in the case of the woofer, either one, or two in series). I also consider including a triple-pole, double-throw switch to retain the option of using the original crossover, should I want to revert to the classic setup in the future. I've created a wiring diagram for this plan, but I’d appreciate it if someone could verify its accuracy:
    View attachment 418625
  • To support an active crossover project, I found several product options. Honestly, I don't really know what would be the easiest for a beginner like me, and has the best bang/buck. I can source cheaply from Chinese Taobao. I plan to use either:
    • A Raspberry Pi running CamillaDSP, combined with a Creative Sound Blaster Omni Surround 5.1 external DAC I have lying around. For amplification, I'm thinking about either:
    • 2x Wondom JAB5 boards hooked up together (master-slave), which have the ADAU1701 DSP built in and 4 amplifier channels each (4x100W). Maybe the neatest and easiest. Don't know about the quality of the DACs and amplification though.
    • I can also use an ADAU1562 DSP board with an AD1938 or CS42448 codec instead of the Raspberry Pi + multichannel DAC. Combined with one of the aforementioned amplification options.
Let me know your thoughts. :)
If you want better bass, without a sub, you don't have much other option, if you want to keep the speaker boxes, then to put another modern 8 inch bass driver in them. BUT it leads to a different appearance of the speakers. You also have to find suitable passive radiator or convert them to sealed or ported, suitable crossover to mids is needed and so on. Nah, it's just too much backward work, hassle (as the farmer who sheared the pig said:that was much cry for little wool :)) Then you might as well buy a couple of other speakers and compare with.

You see, no matter how you twist and turn it with your requests for bass below 50 Hz, you end up back at square 1, as pointed out by several others: Add a subwoofer.:)

Edit:
You liked the idea of having two bass boxes, subwoofers in the same style/looks like your speakers as a stand but you didn't have the opportunity to build them yourself. Proposal:Let some local small carpentry saw plywood boards, plus holes for mounting sub drivers, elements. Then glue and or screw the pieces together yourself? Surface finish that matches, easiest if you're lucky some plastic veneer that looks like your speakers that you just put on the boxes.
 
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Cheers to the new year. Thanks again everyone for your advice. I did some more research these holidays and I'd like to share where I'm at right now and respond to some of your comments.


Yes, what you interpreted is correct.

I obtained two defective pairs of 22AH468 for cheap. I managed to fully repair one pair. The mids didn't work, but were easily fixable following this writeup. It's a common defect with these units. The second pair has one non-repairable blown mid and someone replaced one of the woofers with a random aftermarket unit, so I can't bring this pair back to original form without additional units sourced from somewhere.


The mids are the same, but the tweeters are the 15 Ohm variant (AD0163/T15), which should be about 3 dB less efficient than the 8 Ohm variant (AD0163/T8) in the 22AH487.

I measured the in-room frequency response of the now working pair 22AH468. They're okay but have a strong v-shaped sound signature. The lows have a lot of weird 'wobbly' distortion with my (admittedly sub-par) Denon UDRA-F07 amp, possibly because of the protective car lamps in the crossover filter. Bass is slightly stronger than the 22AH487s but not deeper without EQ. Probably because the 35 L box is too small for the two 8-inch AD8067/W4E woofers.

Overall, not impressed with the sound signature. And I don't dig their box design, which was my main motivator for starting this project on modifying the 22AH487 speakers.

----------

To end the speculation on what combination of drivers is most effective, I’ve decided to seek concrete answers by learning how to measure T/S parameters myself. I've researched the process, ordered the necessary components, and plan to conduct my own measurements of all the drivers with REW. Once I have the data, I'll use recommended speaker design software to evaluate whether a PR system would be effective for the AD8067/W4E, or if it would be better to simply use dual woofers with additional power compensation for lower frequencies. I can use the drivers sourced from the (defective) 22AH468 units for this.

----------

Since I'm already planning to modify the speakers anyways, I want to go all the way with improving the sonic quality of them. I.e. I've already found that the driver break-up modes are not suppressed and I doubt the drivers are phase aligned on the baffle.
  • To address panel vibrations, I’ve ordered bitumen sheets (instead of adding bracing, as I want to preserve the internal volume of the enclosures). I’ve also ordered felt for lining the walls and glass wool to dampen the back wave of the woofer(s). The goal of this isn’t to achieve deeper bass but to improve its quality, like alex-z said. The original synthetic dampening material in the 22AH487s has completely degraded and needs to be replaced anyways.
  • Additionally, I plan to follow through with an active DSP crossover for its inherent advantages and as a personal learning opportunity. To support this, I’ll add a 6-pin DIN socket, allowing individual amplifier channels to connect to each driver (or in the case of the woofer, either one, or two in series). I also consider including a triple-pole, double-throw switch to retain the option of using the original crossover, should I want to revert to the classic setup in the future. I've created a wiring diagram for this plan, but I’d appreciate it if someone could verify its accuracy:
    View attachment 418625
  • To support an active crossover project, I found several product options. Honestly, I don't really know what would be the easiest for a beginner like me, and has the best bang/buck. I can source cheaply from Chinese Taobao. I plan to use either:
    • A Raspberry Pi running CamillaDSP, combined with a Creative Sound Blaster Omni Surround 5.1 external DAC I have lying around. For amplification, I'm thinking about either:
    • 2x Wondom JAB5 boards hooked up together (master-slave), which have the ADAU1701 DSP built in and 4 amplifier channels each (4x100W). Maybe the neatest and easiest. Don't know about the quality of the DACs and amplification though.
    • I can also use an ADAU1562 DSP board with an AD1938 or CS42448 codec instead of the Raspberry Pi + multichannel DAC. Combined with one of the aforementioned amplification options.
Let me know your thoughts. :)
Crossover via DSP, so you can choose the crossover point yourself plus measure FR with an microphone. With that, changes can definitely happen. Measure each driver and see how they behave. An interesting aspect is how the midrange FR looks higher up in frequency, so maybe you can change the crossover point for the better. Perhaps. Maybe not, but you'll notice when you test. :)

One thing you should think about is C-C in relation to the crossover point between mid and tweeter. See what the creator of VituixCAD Loudspeaker simulator
Screenshot_2025-01-04_235130.jpg

....Kimmosto writes about it here:

Minimum c-c is 1.0 x wave length and maximum about 1.4 x wave length at XO frequency assuming that design is conventional uni-directional box (not open baffle) with phase matching (acoustical 4th order) slopes. Good and quite flexible initial/design value for c-c is 1.2 x wave length at XO, giving smooth combination of power and early reflections i.e. balanced sound without significant power dip at XO due to bump in DI and dip in vertical early reflections. In other words, this concept aims lobe nulls to directions which are the least significant for power response and vertical early reflections - and listener sitting in sweet spot of course.

 
(instead of adding bracing, as I want to preserve the internal volume of the enclosures
Use like 1/2"x1/2" square dowels, the impact on internal volume would be about nothing. Round dowels can work, but square dowels give the option to glue together in the middle. For instance front-to-back, side-to-side, top-to-bottom all touching and glued together in the center. Especially if the enclosures are kind of thinwall this will help a lot. Cool project!

Now if you stuffed the enclosure (maybe 1.3 pounds fiberglass per cubic foot of enclosure...Vance Dickason's seminal Loudspeaker Cookbook has a whole chapter) that both absorbs more midrange internally and can make the enclosure appear larger internally. That will shift the tuning a tad lower, and if you added some mass to the passive radiator it would shift lower some more. That should result in some more low frequency output, though way below 50 Hz I dunno about that. And if you're changing drivers then it's a whole different story.
 
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